I have the settings in place but it does not make any difference.
I believe the 301 is pointing to the mainpage? If so it is a joomla only generated page and is being displayed fine. Al the other pages are whmcs links which are not rendered within the template.

Mischa1 had multi-language enabled in Joomla, but did not have it also enabled in the J!WHMCS Integrator settings under the Language Settings. Once that was enabled, the system was able to retrieve the site as expected.